Electric filter arrangement



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Mmm W, WM., W. Mmmm ELEGTRI G FILTER ARRANGEMENT Filed sept. 1, 1%@

Patented Mar. 18, 1941 UNITED STATES meterme rrr-urna annancnresnrWolfgang Hagen, Berlin, Germany, assigner to C'. LorenzAktiengesellschaft, Berlin-Tempelhof,

Germany, a company Application September 1, 1938, Serial No. 227,911 InGermany September 3, 1937 4 claims.

It is well known to transfer broadcast programmes, alarm. signals andthe like over wire networks, such as telephone network, by means of highfrequency carrier currents. In systems of this kind electric filtercircuits must be provided in order to separate the currents of.different frequencies. For instance, the low frequency telephonetalking currents are to be separated in this way from the high frequencywire radio currents.

The usual electric filter circuits comprise a highpass lter and alow-pass filter. The highpass filter is to be of the least possibleattenuation in relation to the high frequency currents while being ofvery great attenuation as regards low frequency talking currents, inorder, for instance, to preclude that the telephone conversations can belistened to on the lines of the high frequency receiving apparatus. Thelow-pass filter on the contrary is to avoid the high frequency currentsarriving in the telephone apparatus in which they would be modulatedwith the talking currents, thus becoming audible in the high frequencyreceiver. Also, both filters must be of a construction symmetric toearth in order that no disturbing voltages shall be received.

Furthermore it is necessary that the number of the constructionalelements employed in the filters and hence the space requirements be aslow as possible.

The invention relates to a filter circuit that shall fulll all theserequirements. The high pass filter here comprises two asymmetric partswhich are arranged in mutual opposition in the lines to which the highfrequency apparatus is connected.

In accordance with another feature of the invention the outputtransformer to which the high frequency apparatus is joined is sodimensioned that its inductivity corresponds to the final inductivity ofthe high-pass filter.

Furthermore the invention'relates to constructional' features of filterarrangements as provided by the invention, the object being to cbtainattenuation values that comply with the aforesaid requirements to thegreatest possible extent.

The invention will be understood from the following description,reference being had to the `accompanying drawing in which- Fig. 1 showsthe circuit arrangement of one embodiment of the invention, while Fig. 2is a partially diagrammatic view representing constructio-nal featuresof this embodiment.

a, b, Fig. 1, denote the supply lines which may be a telephonesuhscribers lines, for instance. V

The electric filter arrangement joined to these lines comprises ahigh-pass filter I and a lowpass filter 2. To lter I a broadcastreceiver or wire radio receiver 3, for example, is connected.A Atelephone set 4 of the customary type is joined to filter 2. Filter Icomprises two asymmetric links, which are constituted by condensers 5,6, an inductance 'I and the inductance of an output transformer 8.Filter I is thus formed of two-` serially connected monominal partswhich are in mutual opposition arranged in the two wires connected toreceiver 3. In the customary highpass filters, composed of two:symmetric parts, four condensers are necessary which are double the sizeof those forming the asymmetric circuit. In. the case of asymmetriclinks, however, there is likewise symmetry to earth, as the condensers5, 6 are practically of the same size, thus having the same capacity toearth. Transformer 8 is so dimensioned that its inductivity correspondsto the final inductivity of lter I, that is to say, is calculated toform a part of this filter. In addition, transformer 8 is so dimensionedthat the ohmic impedance at the receiver terminals, measured with lowfrequency, is very low in order to preclude the low frequencyconversations being iniiuenced from the high frequency side. Filter 2consists of three inductances or coils I0, II, I2, arranged in the lineso, b, and also comprises a condenser I3, connected in bridge of theselines. Inductance I I has two windings disposed on the same core.

It is in general necessary to comprise the individual parts of filterarrangements of this kind in the smallest possible space, so that suchparts will be easily influenced by electrostatic or magnetic fields.Such electrostatic influence may be obviated by means of earthed screenswhile in accordance with the invention the magnetic influence issuppressed by providing for a suitable mutual position of the parts.

As shown in Fig. 2, transformer 8 is so arranged that it shall be cut bythe stray lines of force at right angles and symmetrically. The coilsl., I0, I2 are disposed in coaxial relation to each other and their axisis normal to the centre of the axis of transformer 8 so that thevoltages produced in this transformer by the stray lines of force shallannui each other. This arrangement acts to prevent stray couplings fromtransferring the low frequency energy to high-pass filter I, this filterbeing given by this means a very high attenuation.

In order that the high frequency attenuation of the low-pass filtershall be as great as possible it is necessary that the coils B, 1, l0,I2 traversed by the high frequency currents, do not influence the coilsll. The coils Il are therefore positioned at right angles to the axes ofthe coils 8, l, I0, l2 so as to be in symmetric relation to the strayfields of all these other coils, distubing voltages produced in thecoils l I therefore annulling each other. If in this Way the coils l,I0, l2 are positioned at right angles t0 the middle of transformer 8,and if this transformer and the coils 1, I0, I2 are located at rightangles to the middle of the coils ll, then the best attenuation isattained.

The inductances employed in the described arrangement are preferablyfitted with cores of nely divided ferromagnetic material.

What is claimed is:

1. An electric filter arrangement, particularly for use in thetransmission of intelligence by high frequency energy on a two conductortelephone line which simultaneously are used for ordinary low frequencytelephone tralc, characterized in that the filter arrangement includes ahigh pass lter comprising, a pair of conductors connected to respectiveones of the conductors of said line, a condenser disposed in each of theconductors of said pair, an inductance bridged between the conductors ofsaid pairs to interconnect oppositely poled terminals of saidcondensers, and a transformer bridged across the terminals of saidconductor pair.

2. An electric filter arrangement according to claim 1, wherein theinductance of said transformer corresponds with the final inductance ofsaid high pass filter.

3. An electric filter arrangement according to claim 1, wherein saidtransformer is of a low ohmic impedance for low frequency energy.

4. A unitary electric filter arrangement for use in a circuit comprisinglines for connection to a low frequency apparatus, other lines joined tosaid first named lines for connection to high frequency apparatus, a lowpass lter including two inductances in said lines first mentioned, ahigh pass filter in said other lines including condensers arranged inrespective ones of said other lines, an inductance bridged across saidother lines to interconnect oppositely poled terminals of saidcondensers, and a transformer bridged across the terminals of saidsecond conductors, said two inductances of the lower pass filter and thesaid inductance of said high pass filter being disposed in coaxialrelation to each other, with their axis normal to the center of the axisof the output transformer, whereby the several elements may be arrangedin a small space without deleterious stray coupling therebetween.

WOLFGANG HAGEN.

